OFDM is a multi-carrier transmission technique, which divides available frequency spectrum of a communication channel into many carriers, often referred to as sub-carriers; and adjacent sub-carriers are orthogonally phased to each other. Each of the sub-carriers is then modulated by a low rate data stream. As the sub-carriers are packed more closely than, for example, in frequency division multiplexing (FDMA), OFDM allows the frequency spectrum to be used more efficiently. In addition, OFDM does not require complex time switching, as in time division multiplexing (TDMA), and therefore does not suffer the overhead associated with time switching methods.
FIG. 1 shows an OFDM system for transmitting and receiving information. A serial stream of data symbols is provided via input 101 to a serial to parallel converter 102 that converts the single data stream to several parallel data streams. An inverse discrete Fourier transform (IDFT) module 104 processes the parallel data streams and produces a corresponding number of orthogonal modulated sub-carriers which are provided to a parallel to serial converter 106. In response, the parallel to serial converter 106 provides a serial data signal to a cyclic prefix adder 108, and the cyclic prefix adder 108 produces a transmit data signal at output 110, and the transmit data signal is transmitted on a communication channel.
With further reference to FIG. 1, a corresponding received data signal on the communication channel is provided to a cyclic prefix remover 114 via input 112. The cyclic prefix remover 114 removes the cyclic prefix from the received data signal and outputs a single stream of data to a serial to parallel converter 116. Resultant signals from the outputs of the serial to parallel converter 116 are provided to a discrete Fourier transform (DFT) module 118, which provides a corresponding plurality of demodulated data streams to a parallel to serial converter 120. A serial data signal is then provided by the parallel to serial converter 120 to an equalizer 135 and to a channel estimator 130, and the equalizer operates with the channel estimator 130 to determine the originally transmitted data from received data, and provide the received data via an output 135.
The cyclic prefix is employed to address distortion in the communication channel. Adding the cyclic prefix comprises repeating the last few samples of each data symbol at its beginning, prior to its transmission. The length of the cyclic prefix should be chosen to be greater than or equal to the duration of the impulse response of the communication channel. This allows equalization of the channel distortion in the frequency domain by using a single tap scalar equalizer for each carrier, independently. However, in order to do this the response of the communication channel needs to be characterized. In practice only an estimate of the communication channel's characteristics is used, hence the need for a channel estimator.
There are several methods of performing channel estimation, these include the following schemes; Pilot Symbol Assisted Modulation (PSAM), Blind Channel estimation, and a coded pilot method. Each of these is briefly described below.
PSAM adds periodic transmissions of known symbols or pilots. Pilots comprise data that is known by both the transmitter and the receiver. Therefore, communicating pilot symbols allows the receiver to determine the difference between what was transmitted and what was received, and thus compensate for any variations in the received symbols that are caused by transmission between the transmitter and the receiver i.e. the communication channel. An estimate of the characteristics of the communication channel is required to provide such compensation across time and frequency domains of the communication channel. When the time and frequency characteristics of the communication channel are varying rapidly, as in mobile communication applications for example, channel estimation must be performed more frequently, hence the need for more pilots to be transmitted in order to maintain reliable communication. Thus, reducing the available bandwidth for data transmission.
FIG. 2 shows a PSAM scheme graphically, where both pilot symbols 202 and data symbols 204 are shown in a three dimensional grid across time 206 and frequency 208 axes, and where the vertical axis represents transmission power 210 of the data and pilot symbols. In PSAM, the pilot symbols 202 are inserted at intervals across time and frequency between the data symbols 204. Consequently, part of the signal energy and bandwidth of the communication channel is used for transmitting the pilot symbols 202. A received data signal in a PSAM scheme is passed through a 2-D Wiener filter, which essentially performs interpolation based on the statistics of the communication channel so as to estimate the characteristics of the communication channel between the pilot symbols 202, i.e. where the data symbols 204 are received. In this way, the data symbols 204 can be correctly recovered by taking the estimated time and frequency characteristics into consideration to provide channel equalisation. One implementation of the PSAM scheme is in terrestrial transmission in digital video broadcasting (DVB-T).
A PSAM scheme provides good channel estimation even when applied to time variant channels. However, when the normalized maximum Doppler spread is high, caused by fast changes in the communication channel characteristics, the frequency at which pilot symbols are required increases in order to track such fast changes. This results in more bandwidth being required for pilot symbols, up to ten percent of the bandwidth of the communication channel, and leaving less of the bandwidth for data traffic.
Blind channel estimation does not use pilots. Instead, the data symbols themselves are used to estimate the communication channel. Consequently, bandwidth of the communication channel is preserved. Several blind channel estimation schemes for OFDM are known, however, their tracking ability in a communication channel whose characteristics change or vary with time, Rayleigh fading time variant channels, for example, have not been as good as that of the PSAM scheme.
The coded pilot method is described in U.S. Pat. No. 5,912,876 by H'mimy where a main signal, comprising a quadrature amplitude modulated (QAM) version of a signal to be transmitted, and a pilot signal, are coded separately and transmitted as part of an OFDM signal. When the OFDM signal is received, the main signal portion is detected and an estimation of the communication channel is determined from the detected coded pilot signal portion. Then the detected main signal and the estimation of the communication channel are used to estimate the signal that was transmitted. The coded pilot method is simple to implement, and the coding enhances the detection of the main and pilot signals, in the consequent channel estimation process.
However, a transceiver using the coded pilot method is necessarily more complicated due to the coding in the transmitter, and detection of the codes in the receiver. In addition, a portion of the bandwidth of the communication channel needs to be allocated to support the transmission of the coded signals, thus reducing the usable portion of a predetermined bandwidth.
Hence, there is a need for a channel estimation scheme that provides good performance in a communication channel having varying frequency and time characteristics, while preserving the usable bandwidth of the communication channel.